If you computed a weighted average of all hedge fund returns, you would get a global contribution to GDP. Since hedge funds often don't publish their returns, and it's well known that there's huge amounts of survivorship bias in the data, it would be very difficult to measure accurately. My feeling though is that the contribution would be close to zero, as most of the strategies they pursue are arbitrage based and are zero sum, not wealth creating (with some known exceptions, such as venture capital etc).

Arbitrage is another way of saying eliminating mispricings. Mispricings or traded assets causes a misallocation of societal resources in ways which are negative to economic growth. You need to think second order here.